The demand for terrestrial communications has increased markedly in recent years. To meet the increasing demand, digital cellular mobile communications systems have been implemented which have increased channel capacity.
Among many available digital mobile communications systems, one which has been implemented widely uses a set of modulation and transmission principles known as spread spectrum transmission. A standard for spread spectrum transmission has been adopted in North America which is known as the Code Division Multiple Access Standard ("the CDMA Standard"). The CDMA Standard defines a direct spread system whereby communication data is modulated by multiplication with a pseudo-random spread code at baseband, then shifted to a transmission frequency which is shared by other transmitters which transmit at different phases, i.e., entry points, of the same spread code. According to the CDMA Standard, mobile telephones demodulate spread spectrum modulated transmissions by multiplying the detected spread spectrum transmission with the pseudo-random spread code at the same phase which was used to modulate the signal. The spread code in CDMA Standard communication systems is a periodically repeating sequence of bits approximately 32 kilobits in length. In existing CDMA Standard systems, each base station transmitter is assigned to modulate outgoing communications for transmission at a particular, distinguishable phase, i.e., entry point of a universally used spread code.
A mobile telephone which operates in accordance with the CDMA Standard demodulates transmissions by maintaining the phase of a locally generated spread code in synchronization with the phase of a spread code used by a base station with which it is in communication. However, existing digital mobile telephones, while capable of maintaining synchronization during normal operation, are subject to communication interruptions when the voltage of the main power source of the mobile telephone falls below operational levels. In such case, even when the main power source voltage is quickly restored again to operational levels, communications which are in progress cannot be resumed because the spread code phase of the received transmission cannot be recovered. A mobile telephone system is disclosed herein which provides for recovery of communications following a temporary interruption of operation such as caused by failure of a main power source voltage of the mobile telephone.
An existing spread spectrum digital mobile telephone will now be described, with reference to FIG. 1, to illustrate the problem overcome by the present invention. In FIG. 1, antenna 1 is used to receive and transmit signals between the mobile telephone and one or more base stations. Modulation/demodulation circuit 2 is used to demodulate a received signal from a base station and to provide a regenerated data signal therefrom. Modulation/demodulation circuit 2 is also used to modulate a data signal for transmission to a base station. System time counter 3 is used to acquire and maintain, from the regenerated data signal, a local system time count to permit synchronization of the mobile telephone with a universal system time count used by the mobile communication system. The system time count is used for timing certain processes, e.g., requests for system access to originate a call through the mobile communication system, and certain operations required to hand over a communication in progress from one base station to another when the mobile station moves between base station transmission zones. System time clock 8 is used to provide a clock signal to operate system time counter 3 for maintaining the locally generated system time count in synchronization with the universal system time.
Spread code generating means 10 is used to acquire initial spread code phase synchronizing information from the regenerated data signal, and to locally generate a spread code in phase synchronization with the spread code phases of received transmissions. The locally generated spread code is output to modulation/demodulation circuit 2 for use in modulating and demodulating transmissions from one or more base stations. The spread code phase clock 11 is used to generate a clock signal to operate spread code generating means 10.
The conventional prior art digital mobile telephone also includes a free running timer 4 for independently maintaining an absolute time count apart from the universal system time count used in the mobile communication system. Spare battery 5 powers the free running timer 4. Microprocessor 6 receives user input commands and generates communication control information for originating and receiving communications. Non-volatile memory 7 stores information such as the identification number of the mobile telephone and subscriber telephone numbers for speed-dialing purposes.
The operation of the conventional prior art mobile telephone will now be described. Upon energizing the mobile telephone through a main power source switch (not shown), a transmission signal from a base station of the mobile communication system is detected at antenna 1. The detected transmission signals are demodulated by modulation/demodulation circuit 2, and used by the spread code generating means 10 to generate a spread code at the same phases as those of the transmission signals. One or more transmission signals from the transmitting base station are used by the mobile telephone to establish an initial system time for initializing system time counter 3. The system time counter 3 is initialized at the same time that the mobile telephone unit acquires a spread code phase to initialize the spread code generating means 10.
From the foregoing described operations of the conventional prior art mobile telephone, it is apparent that in order to maintain the locally generated system time count of system time counter 3 in synchronization with the universal system time, the system time clock 8 must be kept operational and in synchronization with the universal system time. In the event that the system time clock 8 is temporarily disabled, the locally generated system time count and spread code will fall out of synchronization and out of phase with the universal system time and the spread code phase used by the transmitting base station. There being no recovery mechanism in the conventional prior art mobile telephone for handling such event, any communication in progress cannot be recovered, even if sufficient power for operating the mobile telephone is restored again quickly.
In the description of the invention to follow, the system time clock 8 is described as becoming temporarily nonoperational or "disabled", when the main power source voltage of the mobile telephone falls below operational level. However, the skilled person in the art will understand the applicability of the specifically disclosed embodiments of the present invention for reestablishing mobile telephone operation following other types of malfunctions which temporarily disable the operation of system time counter 3 or spread code generating means 10.
Accordingly, it is an object of the present invention to provide a mobile telephone unit which maintains a communication in progress despite an event which causes a system time counter or a spread code generating means of the mobile telephone unit to be temporarily non-operational.
Another object of the present invention is to provide a system and method in a mobile telephone for recovering, after a temporary interruption of operation, a system time count and a spread code phase used to synchronize mobile telephone operations with the mobile communication system.